Background:

I have a Dobsonian mounted 8", which I enjoy using very much. I use it mostly for Deep Space Objects (DSO's), but like to do a little planetary observing now and then. Bumping the scope is easy enough, even at 200x, but to draw planetary detail it becomes awkward - bump-sketch-bump-sketch... It would be nice to be able to track, at least visually, so I could keep my hands free for notes and sketches.

While reading The Backyard Astronomer's Guide (Dickenson & Dyer), I saw mention of a "Poncet table" that allows a Dob to track, but from the picture I couldn't figure it out. So I did what any 21st century boy would do, I googled it. Look up "equatorial platform" and you will find scads of information. There is also a Yahoo group dedicated to EQ platforms which has loads of helpfull information, as well as helpful people.

Theory:

Basically, an EQ platform rides on a big arc at one end, and a small ark at the other, so that it rotates about an inclined axis. If this axis is aligned with the pole, you have the ability to track.This sounds technical, perhaps the images below will help.

Begin with two disks of unequal size. Aligned like this, the axis of rotation (yellow line) is about 45 degrees (I am at about 44 degrees lattitude)

Now chop it down along the white dotted line, and drop a board on it, as shown below.

Place your Dobsonian-mounted scope on it, and you are ready to go.

And here is the finished product!

Design

There are a variety of designs, involving two sectors (disk sections) or one sector and a pivot (axial bearing), and some based on conic surfaces. For the discussion below I will focus on the type that I have constructed, which has a single north sector and an axial south bearing.

The first thing to do is determine the centre of mass of your scope and base. The reason for this is you want to make sure that centre lies below the axis of rotation of your platform for stability. If the CM is above the axis, it will have a tendency to tip over. Generally, this is bad. The axis of rotation does NOT need to go through the optical axis of you scope. First of all, you rarely point the scope directly at Polaris, and second of all, if you think about it, you are several thousand kilometres from the Earth's axis of rotation, a few centimetres here or there will not make a difference on an astronomical scale!

To find (approximately) the centre of mass (CM) of your scope and base, try the following:

1. Weigh your base and optical tube separately on a bathroom scale

2. Approximate the centre of mass of the base by picking it up by the handle or side supports to see how it tips - the CM will always lie directly below the sopt where it is suspended from. Use a small piece of masking tape or some other non-permanent method of marking the approximate location of the CM on one of the side supports.

3. The CM of the scope and base together lies along a line between the CM of the base and the pivot point of the scope (ie the centre of the side bearings of the scope when mounted). How far along the line? Starting from the CM of the base, use the formula (length of line x weight of tube)/(weight of base + weight of tube)

Sketch your scope on a shee of graph paper, and then draw a line just above the combined CM, tilted by an angle equal to your lattitude, or at least some value close to your lattitude that will be easy to work with (I am at 43.8 degrees lat, but 45 deg is easier to cut on a table saw!). This will form the axis of rotation of your platform. Draw the platform top under your scope, and then figure out where your bearings need to be, keeping in mind that the bearings must be perpendicular to the axis of rotation.